In recent years, there has been increased interest in the use of pervaporation membrane separation techniques for the selective separation of organic liquid mixtures because of its high separation efficiency and flux rates coupled with potential savings in energy costs. Representative prior art patents describing such procedures are U.S. Pat. Nos. 3,773,844, 3,784,620 and 3,819,742.
In pervaporation, the liquid mixtures feed is provided in contact with one side of a dense non-porous membrane and, after diffusing through the membrane, is removed from the downstream side in the vapour phase under vacuum or swept out in a stream of inert carrier gas and then condensed. The technique depends on the fact that certain membranes permit easier passage of one liquid than another, thereby changing the composition of the mixture by pervaporation through the perm selective membrane.
The permeability of liquids through polymer membranes may be considered as a composite term which is the product of the solubility and diffusion constants. Polymer membranes can exhibit selectivity on the basis of these two parameters, for example mixtures of liquids with different diffusion constants can be separated if their solubilities are different and vice versa. Accordingly, in a pervaporation membrane process, it is desirable to have a polymer film which combines the characteristics of high permeation flux coupled with good selectivity.
Various methods for preparing polymer membranes for various separation processes with good permeability and selectivity to inorganic and liquid mixtures have been described previously. These procedures include copolymerization, graft and block copolymerization, annealing and mechanical stretching, and the use of thin film composite membranes. In this regard, reference is made to the work "Synthetic Polymer Membranes, A Structural Perspective" (3rd Edition), by R. G. Kesting (1985), and to U.S. Pat. Nos. 3,672,975, 3,720,717, 3,930,990, 4,247,665, 4,547,530 and 4,590,098.